1. Field of the Invention
The present invention relates to a device and method for the control of the motion of particles suspended in a gas stream and, more particularly to a device and method for real-time, on-line sampling of particles in process gas streams.
2. Discussion of the Background
In a substantial number of industrial, manufacturing, and scientific processes, the detection, control and analysis of micron and submicron sized particulates is essential. Specifically, the removal of particles, detection of particles for particle counting, and/or sampling of particles for chemical analysis is often crucial to achieving maximum operating efficiency and to minimizing yield losses.
In the semiconductor industry, for example, particle contamination during VLSI manufacturing processes is one of the primary sources of yield loss. About 50% of all yield losses in semiconductor device fabrication are due to particles (i.e., contamination of wafers with particles). One source of particles during semiconductor device manufacture is vacuum processing equipment. This is due to the nature of the mechanical movements (e.g., wafer transfer) in the equipment, and chemical and physical deposition and etching processes which can form particles. Thus, semiconductor manufacturers require a device and method for the removal of particulate matter suspended in process gas streams that can also be utilized in a vacuum environment. Further, the device must have the capability of operating on gas streams having very low particle concentrations (e.g., 5 to 10 particles per cubic foot) and/or very high gas flow rates (e.g., 1 to 5 cubic feet per minute), which are characteristic of semiconductor process gas streams. Producers of specialty gases and researchers in aerosol science and technology represent others interested in particle contamination problems.
In addition to the actual removal of the particles that can potentially contaminate gases and the products being manufactured, sampling of particles present in gases and during product manufacturing provides an effective tool for evaluating the severity of particle contamination and probable yield loss, and for determining the likely source of the particles through particle analysis. In the case of sampling of particulate matter, it is highly desirable to perform such sampling on an unaltered gas/particle sample, during the actual manufacturing process (i.e., in real-time). Further, a sampling device that has the capability to be used on-line, with the processing equipment being operated, in both atmospheric and subatmospheric (i.e., vacuum) conditions, is also desirable.
Devices and methods have been developed for the removal and sampling of particulate matter suspended in gas streams in an attempt to minimize the presence of particles during processing and to isolate or capture particles for particle counting and analysis. Such attempts are characterized, generally, by providing (1) a means for aerodynamic focusing of the particles suspended in the gas stream or (2) a means for creating an electric field to apply a force on the suspended particles causing the particles to collect on a collecting plate or migrate to a designated area.
Available particle control and measurement devices and methods of the aerodynamic focusing type, such as Model HS-LAS Optical Particle Counter, manufactured by Particle Measuring Systems, Inc. of Boulder, Colo. are characterized by the use of an aerodynamic focusing inlet comprising (1) a sampling pipe having an inlet end, an outlet end, and a reduced diameter center portion, and (2) a sheath air inlet. The particle-enriched gas stream is introduced into the inlet end of the sampling pipe. After entering the reduced diameter center portion of the sampling pipe, the particle-enriched gas stream is surrounded by a sheath air stream, introduced through the sheath air inlet, flowing concentrically to the flow direction of the particle-enriched gas stream. The concentrically flowing sheath air stream causes the particles suspended in the particle-enriched gas stream to focus into a narrow axial region in the center of the sampling pipe. Once focused, the particles can be captured and removed, counted, or analyzed. Due to their reliance solely on aerodynamic forces to focus the suspended particles, devices and methods of the aerodynamic focusing type cannot effectively operate in subatmospheric conditions.
Many of the particle control devices and methods currently available utilize a means for creating an electric field to apply a force on particles in a gas stream to cause the particles to collect on a collecting plate or to migrate to a designated area. Such devices and methods are characterized by the use of a pair of electrodes connected to a direct current voltage supply for generating a uniform electric field. In such devices and methods, the particles are removed from the gas stream by (1) electrostatic charging in a unipolar manner of the particles, causing a migration of the charged particles to an oppositely charged electrode, and (2) removing the charged particles from the collecting electrode in a mechanical manner by rapping or vibrating the collecting electrode, such as, for example, in U.S. Pat. No. 3,827,217 to Volsy. Although such devices and methods are somewhat effective in separating the charged particles from the gas stream, the removal of the particles deposited on the collecting electrode by scraping, sweeping, rapping, or vibrating, causes a number of the particles already separated to become re-entrained into the gas stream. Further, due to the collection of the particles on the plate, and subsequent removal step, devices and methods of the collection/removal type do not provide an unaltered particle/gas sample and cannot be operated to perform real-time, on-line sampling for particle counting and analysis. In addition, such devices and methods are not effective for removing electrically neutral particles from the gas stream.
Other devices and methods utilizing a means for creating an electric field to apply a force on particles suspended in a gas stream are characterized by the use of a plurality of electrodes arranged in a specific configuration to drive the particles in the gas stream toward a particular area or region. For example, the device and method for the removal of particles from a gas stream by means of an electric field disclosed in U.S. Pat. No. 4,734,105 to Eliasson et al. includes an electrode configuration having step-wise decreasing electrode spacing between successive direct current field electrode pairs. An untreated gas stream loaded with particles enters the device through an inlet end and is ionized by the electrode pairs and electrically charged in a bipolar manner. A uniform electric field forces the charged particles progressively towards the center of the successive electrode pair configuration, wherein the gas stream loaded with particles is diverted off and withdrawn. Although such devices and methods provide improved particle removal relative to systems utilizing the collection/removal process, their ability to effectively remove and accurately control the motion of particulate matter suspended in a gas stream is still problematic and extremely limited in application. Prior devices and methods cannot adequately transport the focused particles through the electric field or electrode configuration due to their reliance soley on aerodynamic forces (i.e., the velocity of the gas stream/particles flowing through the field), which forces are typically not present in vacuum conditions. Further, the electric field generated by a device of this type does not create sufficient force to focus the particles suspended in gas streams having very low particle concentrations and/or very high gas flow rates.
Other devices and methods for the removal of particulates in gas streams utilizing a plurality of electrodes arranged in a specific configuration attempt to provide greater control (i.e., focusing) of particle movement by a four electrode configuration connected to an alternating current voltage supply to generate a nonuniform, alternating electric field. U.S. Pat. No. 3,496,701 to Berg shows such an arrangement of four electrodes connected to an alternating current voltage supply. The non-uniform, alternating electric field generated includes a point of zero potential and zero electric field in the center of the configuration. The particulates suspended in the gas stream will concentrate at that point. Although such a system provides improved particle control and particulate focusing over the previously described devices and methods, the electric field generated by a Berg-type device lacks sufficient strength to focus the particles suspended in gas streams having very low particle concentrations and/or very high gas flow rates. In addition, a Berg-type system does not have the capability to operate in varying atmospheric conditions (i.e., atmospheric and subatmospheric) due to its reliance soley on aerodynamic forces (i.e., the velocity of the gas stream/particles flowing through the field) for particle transport and the lack of real-time AC voltage adjustment in response to changing pressures.
Notwithstanding the available devices and methods for particle control, there is a need for a particle control device and method that can operate in changing atmospheric conditions (i.e., atmospheric and subatmospheric), on gas streams having very low particle concentrations and/or very high gas flow rates, that can provide for particle removal, detection or analysis on-line, in real-time.
SUMMARY OF THE INVENTION
Accordingly, an important object of the present invention is to overcome the deficiencies of the prior art described above by providing a device and method for the control of the motion of particles suspended in a gas stream that can effectively operate in atmospheric and subatmospheric (i.e., vacuum) conditions.
Another key object of the invention is to provide a particle control device and method having the capability of effectively operating on gas streams having very low particle concentrations and/or very high gas flow rates.
Still another object of the present invention is to provide a particle control device and method that can be operated on-line and can provide for particle removal, detection or analysis in real-time.
Another object of the present invention is to eliminate reliance on aerodynamic forces for controlling the motion of the particles suspended in a gas stream for performing particle focusing and, in subatmospheric conditions, for performing particle transport.
Yet another object of the invention is to provide a particle control device and method having the capability of generating an electric field of sufficient strength to focus the particles suspended in gas streams having very low particle concentrations and/or very high gas flow rates.
Another object of the invention is to provide a device and method for controlling the motion of particles suspended in a gas stream by applying an inhomogeneous electric field generated from an AC field and a DC field to the gas stream.
Still another object of the invention is to provide a device and method for particle control including a plurality of electrodes, wherein at least two electrodes of the plurality of electrodes are connected to an AC voltage supplying means and at least two electrodes of the plurality of electrodes are connected to a DC voltage supplying means.
Another object of the invention is to provide a device and method for particle control including a plurality of electrode sets in series, each electrode set comprising a first pair of opposed electrodes connected to an AC voltage supplying means for oppositely charging the first pair of opposed electrodes and a second pair of opposed electrodes connected to a DC voltage supplying means for oppositely charging the second pair of opposed electrodes.
Yet another object of the invention is to provide a device and method for controlling the motion of particles suspended in a process gas stream, wherein the particles are electrically neutral.
These and other objects are achieved according to the present invention by providing a new and improved device and method for controlling the motion of particles suspended in a gas stream which includes a plurality of electrodes, positioned within a pipe, wherein at least two electrodes of the plurality of electrodes are connected to an AC voltage supply and at least two electrodes of the plurality of electrodes are connected to a DC voltage supply, for generating an inhomogeneous electric field to cause the particles suspended in the gas stream to concentrate into a narrow axial region in the center of the pipe.
In a preferred embodiment, the present invention provides a device and method for controlling the motion of particles suspended in a gas stream, including a pipe, through which at least a fraction of the gas stream flows, a source of AC voltage, a source of DC voltage, and a plurality of electrode sets positioned within the pipe in series. Each electrode set includes a first pair of opposed hyperboloidally shaped electrodes connected to the AC voltage source for oppositely charging the first pair of opposed electrodes and a second pair of opposed hyperboloidally shaped electrodes connected to the DC voltage source for oppositely charging the second pair of opposed electrodes. The first and second pairs of opposed electrodes are positioned at spaced apart intervals around the circumference of the pipe. The electrodes of the first pair of opposed electrodes are spaced apart a predetermined distance and disposed on a first axis in a plane, and the electrodes of the second pair of opposed electrodes are spaced apart at approximately the same predetermined distance and disposed on a second axis, which is perpendicular to the first axis and in approximately the same plane. The first and second pairs of opposed electrodes define an opening through which at least a fraction of the gas stream flows. The plurality of electrode sets are positioned within the pipe in series such that the gas stream flows through the openings defined by each successive electrode set. The spaced apart predetermined distance between the electrodes of the first and second pairs of electrodes of each successive electrode set is decreased to reduce the size of the openings defined by each successive electrode set causing the particles suspended in the gas stream to concentrate into successively narrower axial regions in the center of the pipe. A plurality of computer operated solenoid controlled valves for controlling the rate of flow of the gas stream through the pipe are positioned within the pipe. Each valve is positioned downstream from and adjacent to one of the openings defined by each successive electrode set. Each valve reciprocates between an open position, wherein the rate of flow of the gas stream through the pipe is generally unaffected, and a closed position, wherein the rate of flow is decreased. A plurality of computer operated switches controls the activation and deactivation of each of the electrode sets in response to the opening and closing of the valves.
Other desirable features of the present invention include (1) a particle charger, positioned upstream from the electrodes, for electrically charging the particles suspended in the gas stream and (2) a means, positioned downstream from the electrodes, for receiving the particles transported through the pipe and concentrated into the narrow axial region in the center of the pipe to perform particle detection or analysis.
Still other and more specific objects and features of the invention may be understood from an examination of the following description of the preferred embodiment of the drawings and invention.